But there are many other planetary processes that either bog down or rev up the Earth's spin. Any large-scale interaction between parts of the Earth, such as ocean winds or movements of the planet's molten iron core against the solid mineral mantle, subtly speeds up or slows down the Earth's rotation. Any process that redistributes lots of mass upward or downward will slow or speed the rotation, much as a spinning ice skater slows or quickens her rotation by drawing in or extending her arms. Evaporation, precipitation, melting, changing wind patterns, earthquakes and volcanic eruptions are among the processes that can do this.
"In recent decades, the Earth's rotation has been speeding up," O'Brian points out. "This is almost certainly a temporary effect, based on weather, climate changes, changes in the crust or flow of magma."
For geophysicist Richard Gross of the Jet Propulsion Laboratory in Pasadena, Calif., even the tiniest changes in Earth's rotation are more than merely interesting. "To navigate a spacecraft through space and get it to Mars, you need to precisely know how Earth is oriented with respect to Mars," says Gross, whose works contributes to JPL's interplanetary navigation. He noted that even microsecond changes in Earth's rotation produce enough of a shift in the Earth-Mars orientation to throw the navigational precision off by a mile or more. And that could mean the difference between putting a Martian lander down successfully on a predetermined flat surface or sending it tumbling into a ravine. To help Gross get better at mastering these subtleties, he and a colleague calculated a few years ago that the Sumatran earthquake of Dec. 26, 2004, repositioned enough of the Earth's mass to snip 6.8 microseconds from the length of the day.